In a label printer of the prior art, some heating elements of a thermal head suffer from an electric disconnection during feed of a paper sheet (print medium) due to friction between the paper sheet and the thermal head or the like in some cases. This electric disconnection of the heating element disadvantageously generates blurry printing by which a portion corresponding to the heating element suffering from the electric disconnection is not printed. In particular, if the electric disconnection is generated in any one of the heating elements of the thermal head corresponding to a print area of a bar code, the corresponding bar code may not be recognized when a user tries to read the printed bar code.
As discussed in Patent Literature 1, a check (inspection) on whether or not there is an electric disconnection in the heating element is performed, for example, by preparing a detection resistor in parallel with each heating element and measuring a voltage of a check terminal connected to one end thereof. If a minute electric current flows through the heating element, and the heating element has no electric disconnection, most of the electric current flows through the heating element. Therefore, a voltage of the check terminal becomes low. In comparison, if the heating element has an electric disconnection, most of the electric current flows through the detection resistor. Therefore, the voltage of the check terminal becomes high. In this manner, if the heating elements to be diagnosed are supplied with electric currents one by one, and the voltage check is performed by using the check terminal, it is possible to diagnose electric disconnections of the heating elements.
A process of diagnosing the electric disconnection of the heating element is carried out while the thermal head does not perform printing, that is, while the heating element of the thermal head faces a gap between labels.